The headquarters of Eco-Age, Livia Firth's consultancy business, are set back slightly from King Street in Chiswick, London, behind a whitewashed facade. Inside there is a large open-plan space, with a giant bird's nest complete with painted eggs near the doorway. It is a prop left over from an event Firth organised last year to launch her first Green Carpet Challenge capsule collection - which involved five high-profile British designers and had the support of the British Fashion Council, Anna Wintour, and Natalie Massenet of Net-a-Porter.

'Managing ethics and aesthetics' is Eco-Age's motto. Firth set up the company with her brother Nicola Giuggioli in 2008. Her husband, the actor Colin Firth, is a partner. They started out with a shop on Chiswick High Road, selling eco-friendly paints, sustainable homewares, bamboo towels and wind-up torches - useful things with minimal impact on the environment. If you were lucky, you might even have been served by Colin. They also ran a consultancy helping businesses be more sustainable. Projects included a solar-powered recording studio for KT Tunstall.

In May last year they stopped the retail business and moved to larger premises to focus on consultancy. The executive chairman is Iain Renwick, formerly the CEO of Liberty, who joined in 2012, and the CFO is Lorna Galbraith-Ryan, who joined in 2009. Giuggioli, who completed a master's in sustainable business at Roma Tre university before moving to London in 2004, is the CEO.

The company's offices look like those of a busy start-up. Firth's desk has bookshelves behind it, and a pinboard to one side is covered in a mix of items that says a lot about her. There is a special report on London fashion shows by Suzy Menkes, from the International Herald Tribune; a picture of the Firths together, torn from a magazine; a glamorous shot of a model in a beautiful green ballgown, along with some other fashion images; and a newspaper spread showing the aftermath of the Rana Plaza factory disaster of last year.

When I visited, Firth, 44, was in the garden at the back of the office being photographed for this feature. She is good at being photographed; it is something she does a lot and is comfortable with. Slim and very neatly put together - dark eyes, chiselled cheeks, hair pulled into a sleek side ponytail - she had chosen two outfits from her wardrobe: a black cocktail dress by Tory Burch for the Green Carpet Challenge ('It was made using bits and bobs left over in her workshop,' Firth told me after the shoot), and a top by Beulah ('Two English girls who do a lot in India, with a cooperative of women') with jeans.

Firth has become an expert in dressing herself stylishly as well as ethically. She is about as far as you can get from the stereotypical hair-shirted, hempseed-eating ecoactivist. Her most important attribute is her ability to play the game with luxury fashion's power circle. As a regular on the red carpet, attending premieres, charity dinners, the Golden Globes and the Oscars with her husband, Firth came up with the idea of using her profile to champion sustainable fashion. For the Venice Film Festival premiere of A Single Man in 2009 she wore a dress by her friend Orsola de Castro, of the upcycling specialist From Somewhere. Firth also sought the help of a friend and mentor, the Observer's ethical-living columnist, Lucy Siegle, and in December 2009 they came up with the idea of the Green Carpet Challenge.

For the first GCC, at the Golden Globes in 2010, Firth had red-carpet reporters slightly mystified when she turned up in a repurposed Christiana Couture wedding dress. For the Paris premiere of The King's Speech, in January 2011, she wore an outfit made from one of Colin's old suits by the east-London upcycling queens Junky Styling. And when Colin won an Oscar the following month she wore a dress inspired by one her grandmother had owned. It was made by Gary Harvey - a relative unknown in international fashion - by patching together 11 vintage frocks.
Above Firth with her husband, Colin, at the 2011 Oscars, the 2012 Oscars and the 2010 Golden Globes, to which she wore a repurposed Christiana Couture wedding dress.

The GCC has, Firth says, given her a real sense of purpose at the various awards ceremonies she attends with Colin (who generally wears vintage Tom Ford tuxedos). Her outfits are debated by the fashion press and style bloggers and she is given the chance to discuss her gowns and her fashion agenda. 'People started talking about it and being curious,' she said. By 2012 other celebrities were joining the campaign. Meryl Streep wore a full-length gold gown by Lanvin made from eco-certified fabrics to the Academy Awards (Firth wore a Valentino dress made using recycled plastic bottles). Michael Fassbender, Javier Bardem and Viola Davis also joined in, as did Cameron Diaz, who wore an organic silk dress by Stella McCartney to the Met Ball in New York. 'When you explain what you are trying to do and how easy it is,' Firth said, 'it's like a no-brainer to do it.'

The GCC became a powerful brand in itself, and last September Firth launched the Green Carpet Challenge capsule collection. She had gathered five of the leading names in British fashion - Christopher Bailey of Burberry, Victoria Beckham, Christopher Kane, Erdem Moralioglu and Roland Mouret - to create items of clothing that were both worthy of the red carpet and produced with enough care for the environment and respect for the people who made them that even the most radical of eco-fashion activists would be proud to wear them.

With a little help from the actress Emma Watson, who modelled the designs for Net-a-Porter, the collection sold out. 'We don't support slave labour in this country, so we shouldn't support those conditions in other countries. I can't wrap my head around why ethical clothing is a speciality and not a base standard,' Watson said in an interview with the website. And the designers involved were keen to embrace a more sustainable supply chain. It was a big moment for Firth's business: sustainably sourced, ethically produced fashion had been given its moment to shine. And that, ultimately, is Livia Firth's mission in life: to change the way our fashion is produced and consumed.

Watch: Handprint, a provocative short film starring Elettra Wiedemann by director Mary Nighy and producers White Lodge, premiered at London Fashion Week at the launch of the Green Carpet Challenge Capsule Collection on Net-A-Porter and aims to raise awareness of the challenges faced by garment workers and jewellery producers around the world.

Above Firth (second from right) at an evening to celebrate the Global Fund, which she hosted with Anna Wintour (fourth from right), Natalie Massenet (second from left) and the Earl and Countess of Mornington, in London last year.

What sets Firth apart from other activists is that American Vogue (and British Vogue, for which she wrote the Green Carpet Challenge blog) can talk to her without feeling their fashion credentials are compromised, as can companies such as Gucci and Burberry. Firth is all about ethics, but not at the expense of aesthetics. She understands the power of glamour and the business that is luxury.

Livia Firth was born in September 1969 in Rome. She was one of four children. Her younger brothers, Nicola and Alessandro (an actor who still lives in Rome), are twins. Her sister, Caterina, is a year older than her and works in finance in Milan. Their father ran a business and their mother stayed at home to look after the children. 'It was a very different society when we grew up,' she said. 'It was different times. Life wasn't so complicated. My mum and dad had no money. We went on holiday [in Italy] in the summer. It was a very normal city upbringing.'

She studied humanities at university in Rome and then stayed on to do a doctorate in cinema. She got a job working with a TV producer and they went to Colombia to work on a series called Nostromo. It was there she met Colin Firth on set. She was 25, he was 35.

'First he had to do that courtship thing with my family,' Firth said. He moved to Rome to be with her. 'He was there for two years. We lived separately - I was a good Italian girl,' she said, laughing. Then they moved to London. That was 17 years ago. The couple's two sons, Luca and Matteo, were born in 2001 and 2003. The family has a holiday home in Umbria, near Firth's parents', where they grow their own fruit and vegetables and produce olive oil. It is, she admits, easier to live a greener life in the country than in the city.

Firth has long been a supporter of Oxfam, but in 2012 she was made a global ambassador for the charity. She is also part of the Circle, a partnership of influential women who use their networks, skills and resources to help Oxfam empower vulnerable women and tackle poverty. 'When people ask me, "When did you become green?" I say, "Green as opposed to what? To red? Yellow?" It is more about active citizenship, it's about taking responsibility. It's about curiosity and ownership. I think we all have a choice to live life passively and let it pass by or to be active.' Colin was, she says, 'politically alert', so she became part of those conversations. And then she started to travel with Oxfam, which really started to motivate her. 'I went to Bangladesh seven or eight years ago,' she said. 'When you see these things you can't shake it out, you can't pretend they don't exist any more.'

On April 24 Firth will speak at the Copenhagen Fashion Summit, the industry's biggest conference on sustainability. She will be joined at the summit by Marie-Claire Daveu, the head of sustainability of the luxurygoods group Kering (which owns Stella McCartney and Gucci, and for which Eco-Age consults), as well as Helena Helmersson, the head of sustainability at H&M. The day will mark the first anniversary of the Rana Plaza disaster in Bangladesh, in which more than 1,100 textile workers were killed when the eight-storey factory they were working in collapsed. Among the retailers producing clothes at the factory were Primark (which recently paid out $10 million in compensation to the families, bringing its total contribution to $12 million as it paid $2 million in short-term support after the disaster), Mango and Walmart.

'I think Rana Plaza changed the history of fashion for ever,' Firth said. 'We can't allow ourselves to forget about that. We can't.'

Firth went with Lucy Siegle to meet garment makers at some factories in Bangladesh on a trip for Oxfam in 2008. 'It is shocking. There is an armed guard at the door so people can't get in and out. Women have two toilet breaks a day. They are producing 100 pieces an hour!

'There is no ventilation,' Firth continues. 'Every window has bars, so if there is a fire people can't get out. If you ask, "What can we do?" they say, "Please don't say anything, we can't afford to lose our jobs." It's modern-day slavery.'Her friend and fellow ethical-fashion campaigner Carry Somers, who runs a fair-trade hat brand called Pachacuti, has organised a global campaign called Fashion Revolution Day to mark the anniversary and ensure that we the consumers make the connection between the clothes we buy and the people who make them - often in appalling and dangerous conditions. On April 24 thousands of people around the world will be wearing clothing inside out to expose the labels, showing where the garments were made, and using social media to ask, 'Who made your clothes?' 'I think we are wearing our T-shirts inside out for Fashion Revolution Day,' Firth said. 'Carry Somers has been amazing to [organise this]. It's huge in every country.'

When we met, in March, Firth was about to travel to the Basel watch fair with the family-owned jewellery company Chopard to launch the first ever gold watch for men made using gold that is fairmined (which means that the mines are overseen by the NGO the Alliance for Responsible Mining). The project began when Firth asked the company's co-president Caroline Scheufele a simple question: where did its gold come from? 'If you ask any jeweller in the world, everyone will tell you, "From the bank." You buy it in ingots, but no one knows where the gold [originates] - not even the bank. So when I asked that question to the president of Chopard, she said, "I don't know… we have to make it better." Immediately she decided to change it.'

In March 2013 Eco-Age launched the GCC Brand Mark, a guarantee of sustainable excellence, which has been applied to the Green Carpet Challenge collection of fine jewellery by Chopard and is stamped into the leather of a range of bags Eco-Age launched in collaboration with Gucci in July 2013. The Gucci for the Green Carpet Challenge collection boasts a transparent supply chain and is made from anti-deforestation leather from cattle reared in the Amazon. Brazil is now the biggest commercial farmer of cattle in the world, contributing to 75 per cent of tropical deforestation, so Eco-Age worked alongside the Rainforest Alliance and the National Wildlife Federation with ranches that do not clear forests for their cattle and tanneries that do not use chemicals.

Firth is quite uncompromising in her vision and sceptical of efforts by fast-fashion companies to 'green' their collections. 'Fast fashion is a phenomenon; you can't make it better. You can use as much organic cotton as you want, but you are still producing one million shorts for their amount of money [too little]. It can't be ethical. You are still producing rubbish at the end. Where do all these clothes end up? They have to slow down. They have to change their core business model.'

What, I asked, about the mother-of-three on a limited income? Where are she and her children supposed to shop? These people are not the problem, she said. What she objects to is people who can afford to buy better-made, better-quality clothes that will stand the test of time wasting their purchasing power on disposable fashion. 'If you do the price-per-wear [calculation], fast fashion doesn't add up any more. It costs much more [per wear] than something that is good quality and costs a bit more. 'And in case you were wondering where to shop, Marks & Spencer is - according to Firth - OK. 'The price point is very different.'

Firth tries to operate a 30-wear rule, which was suggested by Siegle, whose book, To Die For: Is Fashion Wearing Out the World?, was published in 2011. 'Eco-Age is all about practical solutions… If you can commit to wearing something 30 times, it's already fantastic, no matter where you bought it,' Firth said. 'Buy with purpose. I have things I wear over and over again. I think I'm one of the few people I know who still mends socks when they have a hole. But it is easier to buy new socks than to mend them, because it's cheap.'

Firth showed me a dress she was planning to wear that night. It was wonderfully bright, in a 1960s abstract floral print. 'That was my mum's going away dress for the day of her wedding; she also wore it when she was pregnant with my sister and me. I wore it when I was pregnant and my sister wore it when pregnant. We're still using it in the family. I just brought it back from Milan because my sister borrowed it for a wedding, and I'm wearing it tonight for an event.' It was made in the 1960s by a seamstress for her mother. 'Isn't it beautiful when you have things like that in your wardrobe that have that sort of story?' Firth asked, her mouth breaking into its infectious smile.

She makes it sound so easy, but it can't always be so straightforward to shop with a conscience - particularly with Luca and Matteo in tow. Firth buys her socks and underwear at Marks & Spencer, like everyone else, and admits to shopping at Westfield when she has to, complaining that the mall has closed down the local independent shops on her high street. 'If you want to buy trainers for your kids, you have to go to Westfield,' she said. 'I hate that, but I have no choice. Someone recently said to me, "You are such an optimist," and I said, "I'm not an optimist." Things have to change… If no one adapts and no one does it, in 30 years' time we'll be done.'

While Firth is not alone in her campaign, she has become a particularly powerful voice. In November 2012 she became a leader for change, a title awarded by the United Nations and Foundation for Social Change conference. As creative director of Eco-Age she instigates projects, working with her team of 25 to improve supply chains and help businesses benefit from increased sustainability. 'Over the last 15 years we've been brainwashed,' she said. 'How do you join the dots, reconnect the consumer with the clothes they buy, make an emotional bond that goes back to the producers, the women and men who make our clothes?' While she does not claim to have the solution to the problem, she is trying to do her bit. 'When I grew up, fast fashion didn't exist,' she said. 'H&M, Zara, Topshop, they didn't exist. So I couldn't buy clothes at that [low] price. I was a student, I didn't have money, and yet I managed to go to my parties, to the discos.' She managed to look smart for her first job with a television producer, and she even managed to catch the eye of Mr Darcy in the process, without so much as a Zara party dress or a quick-fix Friday-night frock.

'What we don't realise is [the fast-fashion chains] actually created poverty.' Firth believes that the fashion industry has gone into overdrive and is producing too much, too quickly. And in the process it has spawned a generation of consumers who buy clothes at an irresponsible rate - clothes that are often worn only once. Some would call it democracy of design. She disagrees. Fast fashion has simply resulted in textile workers in Bangladesh, Cambodia and other parts of the world being forced to work overtime for too little money. 'We've got too much, then we find ourselves in this chaos,' she said. 'Let's slow down.'